Semiconductor memory device including multi-layer gate structure

ABSTRACT

A semiconductor memory device includes a first select transistor, first stepped portion, and a first contact plug. The first select transistor is formed on a side of an upper surface of a substrate and has a first multi-layer gate. The first stepped portion is formed by etching the substrate adjacent to the first multi-layer gate of the first select transistor such that the first stepped portion forms a cavity in the upper surface of the substrate. The first contact plug is formed in the first stepped portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Applications No. 2001-352020, filed Nov. 16,2001; and No. 2002-156982, filed May 30, 2002, the entire contents ofboth of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor memory apparatus,particularly to a semiconductor integrated circuit apparatus including anonvolatile memory cell transistor and select transistor. Moreover, theapparatus is used, for example, in a semiconductor memory or memoryembedded device which includes a NAND type memory array.

2. Description of the Related Art

In recent years, an electrically erasable and programmable read onlymemory (EEPROM) which is electrically writable/erasable has remarkablyspread. A conventional structure of EEPROM will be described withreference to FIG. 1. FIG. 1 is a sectional view along a bit linedirection in a memory cell array region of a NAND type flash memory.

As shown, two select transistors ST1, ST2 and n memory cell transistorsMC1 to MCn connected in series between the select transistors are formedin a memory cell array. Each of the transistors ST1, ST2, MC1 to MCnincludes a multi-layer gate formed on a silicon substrate 100 with agate insulating film (tunnel insulating film) 110 interposedtherebetween. The gate insulating film 110 is thin to such an extentthat a tunnel current can flow. The multi-layer gate includes: a chargeaccumulation layer 120 electrically separated for each memory cell; acontrol gate 130; a inter-gate insulating film 140 formed between thecharge accumulation layer 120 and control gate 130; and a gate cap film150 disposed on the control gate 130. In the silicon substrate 100 onboth sides of the multi-layer gate, n-type impurity diffusion layers 160having a conductive type opposite to a type of the silicon substrate 100in which a channel region is formed are formed. The impurity diffusionlayer 160 functions as a source, drain region. The selection and memorycell transistors are formed including the multi-layer gate and impuritydiffusion layer 160. Moreover, two select transistors ST1, ST2 and nmemory cell transistors MC1 to MCn constituted as described above areconnected in series and disposed so that the impurity diffusion layer160 is shared.

An insulating film 170 is formed on the multi-layer gate, and a contactbarrier film 180 is formed on the insulating film 170. Moreover, aninterlayer insulating film 190 is formed on the contact barrier film180. Further in the interlayer insulating film 190, a bit line contactplug 200 and common source line contact plug 210 are formed to reach thedrain and source regions of the select transistors ST1, ST2.Additionally, a metal wiring layer 220 is formed on the interlayerinsulating film 190. A part of the metal wiring layer 220 is connectedto the common source line contact plug 210, and functions as a commonsource line. Furthermore, an interlayer insulating film 230 is formed onthe interlayer insulating film 190, and a metal wiring layer 240 isformed on the interlayer insulating film 230. The metal wiring layer 240is connected to the bit line contact plug 200 via a contact plug 250,and functions as a bit line. Additionally, the charge accumulation layer120 and control gate 130 of the select transistor are electricallyconnected in a region (not shown).

In the flash memory constituted as described above, the chargeaccumulation layer and semiconductor substrate transmit/receive electriccharges with each other via the gate insulating film, so that data isrewritten.

However, in the conventional semiconductor memory device, when thenumber of rewritings of data increases, the electric charge is trappedin the gate insulating film. Then, the data is reversed by de-trappingthe trapped electric charge, and reliability of the memory cell isdeteriorated. Particularly, in the flash memory of a type such that thesemiconductor substrate is positively biased with respect to the chargeaccumulation layer and thereby the data is rewritten, this tendency isremarkable. In this type of flash memory, electrons are discharged fromthe charge accumulation layer using Fowler Nordheim (FN) tunnel current.As a result, the data is rewritten. In this case, an electric field isconcentrated particularly in an edge of the charge accumulation layer.Therefore, as compared with the gate insulating film on a channelregion, in the gate insulating film in the vicinity of the chargeaccumulation layer edge, electron trap easily occurs. Furthermore, whena gate length shortens with miniaturization of a semiconductorapparatus, there is a tendency of an increase of an influence in thevicinity of the charge accumulation layer edge with respect to thechannel region. As a result, the deterioration of reliability of thememory cell becomes more remarkable.

Moreover, as shown in FIG. 1, the bit line contact plug 200 and commonsource line contact plug 210 are formed in a self-aligning manner withrespect to the gate electrodes of the select transistors ST1, ST2. Inthis case, short circuit tends to easily occur between the gateelectrode 120 and bit line contact plug 200 of the select transistorST1, or between the gate electrode 120 and common source line contactplug 210 of the select transistor ST2.

BRIEF SUMMARY OF THE INVENTION

A semiconductor memory device according to a first aspect of the presentinvention comprises:

-   -   a first select transistor formed on a side of an upper surface        of a substrate and having a first multi-layer gate;    -   a first stepped portion formed by etching the substrate adjacent        to the first multi-layer gate of the first select transistor        such that the first stepped portion forms a cavity in the upper        surface of the substrate; and    -   a first contact plug formed in the first stepped portion.

A memory card according to a second aspect of the present inventioncomprises:

-   -   the semiconductor memory device according to the first aspect of        the present invention.

A card holder according to a third aspect of the present inventioninserts the memory card according to the second aspect of the presentinvention therein.

A connecting device according to a fourth aspect of the presentinvention inserts the memory card according to the second aspect of thepresent invention therein.

A device for storing information according to a fifth aspect of thepresent invention comprises:

-   -   a memory card which includes a semiconductor memory,    -   the semiconductor memory including: a select transistor formed        on a side of an upper surface of a substrate and having a        multi-layer gate, a stepped portion formed by etching the        substrate adjacent to the multi-layer gate of the select        transistor such that the stepped portion forms a cavity in the        upper surface of the substrate and a contact plug formed in the        stepped portion.

A system for accessing a storage medium according to a sixth aspect ofthe present invention comprises:

-   -   a memory card which includes a semiconductor memory,    -   the semiconductor memory including: a select transistor formed        on a side of an upper surface of a substrate and having a        multi-layer gate, a stepped portion formed by etching the        substrate adjacent to the multi-layer gate of the select        transistor such that the stepped portion forms a cavity in the        upper surface of the substrate and a contact plug formed in the        stepped portion.

An apparatus for storing information according to a seventh aspect ofthe present invention comprises:

-   -   memory means incorporated within the memory card for storing        information; the memory means including a select transistor        being formed on a side of an upper surface of a substrate and        having a multi-layer gate, a stepped portion formed by etching        the substrate adjacent to the multi-layer gate of the select        transistor such that the stepped portion forms a cavity in the        upper surface of the substrate and a contact plug formed in the        stepped portion;    -   input means for inputting information to be stored in the memory        means; and    -   memory reading means for reading information stored in the        memory means of the memory card.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a sectional view of a conventional NAND type flash memory;

FIG. 2A is a plan view of a semiconductor memory device according to afirst embodiment of-the present invention;

FIG. 2B is a sectional view along a line 2B-2B in FIG. 2A;

FIGS. 3A to 3C are sectional views successively showing manufacturingsteps of the semiconductor memory device according to the firstembodiment of the present invention;

FIG. 3D is an enlarged view of a partial region of FIG. 3C;

FIGS. 3E to 3F are sectional views successively showing themanufacturing steps of the semiconductor memory device according to thefirst embodiment of the present invention;

FIGS. 4A and 4B are sectional views of the semiconductor memory deviceaccording to a second embodiment of the present invention;

FIG. 5 is a sectional view of the semiconductor memory device accordingto a third embodiment of the present invention;

FIGS. 6A and 6B are sectional views successively showing themanufacturing steps of the semiconductor memory device according to thethird embodiment of the present invention;

FIG. 6C is an enlarged view of the partial region of FIG. 6B;

FIGS. 6D and 6E are sectional views successively showing themanufacturing steps of the semiconductor memory device according to thethird embodiment of the present invention;

FIG. 7A is a sectional view of a memory cell included in thesemiconductor memory device according to the third embodiment of thepresent invention, and a diagram showing an electric field distributionin the vicinity of a gate insulating film;

FIG. 7B is a sectional view of the memory cell included in theconventional semiconductor memory device, and a diagram showing theelectric field distribution in the vicinity of the gate insulating film;

FIGS. 8A to 8C are sectional views successively showing themanufacturing steps of the semiconductor memory device according to amodification example of the third embodiment of the present invention;

FIG. 9 is a sectional view of the semiconductor memory device accordingto a fourth embodiment of the present invention;

FIG. 10A is a plan view of the semiconductor memory device according toa fifth embodiment of the present invention;

FIG. 10B is a sectional view along a line 10B-10B in FIG. 10A;

FIG. 11 is a sectional view of the semiconductor memory device accordingto a sixth embodiment of the present invention;

FIG. 12 is a sectional view of the semiconductor memory device accordingto a seventh embodiment of the present invention; and

FIG. 13 is a sectional view of the semiconductor memory device accordingto an eighth embodiment of the present invention.

FIG. 14 is a block diagram showing an illustrative internal structure ofa memory card in accordance with an embodiment of the present invention.

FIG. 15 is a block diagram showing an illustrative internal structure ofa memory card in accordance with an embodiment of the present invention.

FIG. 16 is an illustrative example of cardholder and a memory card inaccordance with an embodiment of the present invention.

FIG. 17 shows a connecting apparatus operable to receive a memory cardor cardholder.

FIG. 18 is an illustrative example of a connecting apparatus connectedto a personal computer via a connecting wire and having a memory cardinserted therein.

FIG. 19 shows an IC card in accordance with an embodiment of the presentinvention.

FIG. 20 is a block diagram of an IC card in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

A semiconductor memory device according to a first embodiment of thepresent invention will be described with reference to FIGS. 2A and 2B.FIG. 2A is a plan view of a NAND type flash memory according to thefirst embodiment, and FIG. 2B is a sectional view taken along a line2B-2B in FIG. 2A.

As shown, a plurality of element isolating regions STI is formed in astrip form in a p-type silicon substrate (or p-type well region) 10.Each element isolating region STI includes a trench disposed in thesilicon substrate 10, and an insulating film such as a silicon oxidefilm (SiO₂) by which the trench is buried. Moreover, a region betweenthe element isolating regions STI disposed adjacent to each other formsa element region AA in which a semiconductor device is to be formed. Athin gate insulating film (tunnel insulating film) 11 through which atunnel current can flow is formed on the whole surface of the elementregion AA.

A plurality of NAND cells is formed in the element region AA. Each ofthe NAND cells includes two select transistors ST1, ST2 and n memorycell transistors MC1 to MCn having current paths (source/drain)connected in series between the select transistors. Each of thetransistors ST1, ST2, MC1 to MCn includes a multi-layer gate disposed onthe silicon substrate 10 with the gate insulating film 11 interposedtherebetween.

The multi-layer gate is formed as follows. That is, a chargeaccumulation layer (FG) 12 separated for each memory cell is formed onthe gate insulating film 11, and a control gate 13 is formed on thecharge accumulation layer 12 with a inter-gate insulating film 14interposed therebetween. Furthermore, a gate cap film 15 is formed onthe control gate 13, so that the multi-layer gate is formed. Forexample, a polycrystal silicon film is used to form the chargeaccumulation layer 12 and control gate 13, a silicon oxide film (SiO₂)is used to form the gate insulating film 11, a multilayered structure ofthe silicon oxide film and a silicon nitride film (ON, NO, ONO films) isused to form the inter-gate insulating film 14, and the silicon nitridefilm (SiN) is used to form the gate cap film 15.

The silicon substrate 10 right under the multi-layer gate forms achannel region in which a channel of each transistor is formed.Moreover, n-type impurity diffusion layers (second semiconductorregions) 16 of a conductive type opposite to that of the siliconsubstrate 10 is formed in the surface of the silicon substrate 10positions on both sides of the channel region. The impurity diffusionlayers 16 function as source, drain regions (S, D) of each transistor.Since the above-described multi-layer gate and impurity diffusion layers16, 16 are disposed, each select transistor and memory cell transistorare formed.

The memory cell transistors MC1 to MCn are arranged so that thetransistors disposed adjacent to each other share the impurity diffusionlayer, and a memory cell unit is formed. The select transistors ST1, ST2are disposed so that the select transistors and the memory celltransistors MC1, MCn on ends of the memory cell unit share one of theimpurity diffusion layers. As described above, the NAND cell is formedincluding one memory cell unit and two select transistors ST1, ST2, anda memory cell array is formed including a plurality of NAND cells.

An insulating film 17 such as the silicon oxide film is formed on themulti-layer gate and impurity diffusion layer 16, and further a contactbarrier film 18 such as the silicon nitride film is formed on theinsulating film 17. An interlayer insulating film 19 such as a boronphosphorous silicate glass (BPSG) film is formed on the contact barrierfilm 18 so that the select transistors ST1, ST2 and n memory celltransistors MC1 to MCn are coated.

In the interlayer insulating film 19, contact holes C1, C2 are formed toreach the drain and source regions of the select transistors ST1, ST2positioned in endmost portions of the transistors connected in series inthe NAND cell. In the contact holes C1, C2, a bit line contact plug 20and common source line contact plug 21 are formed. The bit line contactplug 20 and common source line contact plug 21 are formed of conductivefilms such as a low-resistance polycrystal silicon film and metalmaterial.

Moreover, a metal wiring layer 22 is formed on the interlayer insulatingfilm 19. A part of the metal wiring layer 22 is connected to the commonsource line contact plug 21, and functions as a common source line (SL).An interlayer insulating film 23 is formed further on the interlayerinsulating film 19, and a metal wiring layer 24 is formed on theinterlayer insulating film 23. The metal wiring layer 24 is connected tothe bit line contact plug 20 via a contact plug 25, and functions as abit line (BL).

The control gate 13 is connected in common between the NAND cellsdisposed adjacent to each other in a direction intersecting with theelement isolating region STI. In a region (not shown), the control gates13 of the select transistors ST1, ST2 are connected to select gate linesSGS1, SGD1, and the control gates 13 of the memory cell transistors MC1to MCn are connected to control gate lines CG1 to CGn. In a region (notshown), the charge accumulation layer 12 and control gate 13 of theselect transistor are electrically connected to each other, and signalsgiven to select gate lines SGS, SGD are directly applied to the chargeaccumulation layer 12. The common source line SL is connected in commonbetween the NAND cells disposed adjacent to each other.

Moreover, the bit line BL is connected to a column selector (not shown),the select gate lines SGD, SGS and control gate lines CG1 to CGn areconnected to a row decoder (not shown), and the common source line SL isconnected to a deletion control circuit (not shown).

In the NAND cell, a part of the surface of the n-type impurity diffusionlayer 16 between the multi-layer gates of the select transistorsdisposed adjacent to each other is removed, as shown in FIG. 2B. Theimpurity diffusion layer 16 has a recess, the bottom of which lies belowthe level of the surface of the silicon substrate 10 right under thegate insulating film. The recess caused the difference in level betweena surface of the silicon substrate 10 right under the gate insulatingfilm 11 and a surface of a part of the impurity diffusion layer 16 shallbe referred to as “stepped portion”, hereinafter. In particularly, thestepped portion in the memory cell array region is defined as “firststepped portion”. In other words, an interface of the surface of thesilicon substrate 10 in which a channel is formed and the gateinsulating film 11 is positioned to be higher than the interface of thecontact plug 20 and drain region 16 by a height of the first steppedportion, in the select transistor ST1. Additionally, in the selecttransistor ST1, the interface of the surface of the silicon substrate 10in which the channel is formed and gate insulating film 11 exists in thesame plane as that of the interface of the source region 16 and gateinsulating film 11. In the select transistor ST2, the interface of thesilicon substrate 10 surface in which the channel is formed and gateinsulating film 11 is similarly positioned to be higher than theinterface of the contact plug 21 and source region 16 by the height ofthe first stepped portion. Furthermore, even in the select transistorST2, the interface of the surface of the silicon substrate 10 in whichthe channel is formed and gate insulating film 11 exists in the sameplane as that of the interface of the drain region 16 and gateinsulating film 11. As described above, the stepped portion is formedonly in the select transistors ST1, ST2, and is not formed in the memorycell transistor.

A manufacturing method of the NAND-type flash memory constituted asdescribed above will next be described with reference to FIGS. 3A to 3F.FIGS. 3A to 3F excluding FIG. 3D are sectional views successivelyshowing manufacturing steps of the flash memory according to the presentembodiment, and FIG. 3D is an enlarged view of a partial region of FIG.3C.

First, as shown in FIG. 3A, the element isolating region (not shown) isselectively formed in the p-type silicon substrate (or p-type well) 10.Thereafter, the gate insulating film 11 is formed on the surface of thechannel region of the element region AA. Moreover, the chargeaccumulation layer 12, inter-gate insulating film 14, control gate 13,and gate cap film 15 are successively formed on the gate insulating film11. Moreover, the charge accumulation layer 12, inter-gate insulatingfilm 14, control gate 13, and gate cap film 15 are patterned in aself-aligning manner so that side wall portions are aligned, and themulti-layer gate is formed as shown in FIG. 3A.

Subsequently, as shown in FIG. 3B, the surface of the silicon substrate10 is coated with a resist 40. Moreover, a photolithography technique isused to remove the resist 40 between the multi-layer gates of the selecttransistors disposed adjacent to (opposite to) each other. That is, thegate insulating film 11 is exposed on a region in which the drain regionof the select transistor ST1 on a drain side, and the source region ofthe select transistor ST2 on a source side are to be formed.Subsequently, the gate insulating film 11 exposed between themulti-layer gates is etched, and the silicon substrate 10 between themulti-layer gates is further etched. As a result, a recess is formed inthe surface of the silicon substrate 10 between the multi-layer gates ofthe select transistor. In this step, while an etching selection ratio isoptimized, the etching of the gate insulating film 11 and siliconsubstrate 10 may continuously be etched in an etching treatment of asilicon oxide film base. Alternatively, after the gate insulating film11 is etched by the etching treatment of the silicon oxide film base,the silicon substrate 10 may also be etched by a silicon-based etchingtreatment. In this case, the etching is sufficiently performed until apart of the gate insulating film 11 and silicon substrate 10 between themulti-layer gates of the select transistor is removed, so that a sidesurface of the multi-layer gate of the select transistor becomesvertical to the surface of the silicon substrate 10.

Subsequently, the resist 40 is removed by ashing. Thereafter, as shownin FIG. 3C, by a thermal oxidation method, the silicon oxide film 17 isformed on the upper surface and side wall of the multi-layer gate and onthe silicon substrate 10 between the multi-layer gates. Furthermore, ifnecessary, the surface of the silicon substrate 10 is coated with theresist, and a resist opening portion is formed only in a memory cellarray region by a photolithography process. Moreover, n-type impuritiesare injected in the silicon substrate 10 between the multi-layer gatesto form the n-type impurity diffusion layer 16 which constitutes thesource and drain regions. By the present step, the select transistorsST1, ST2 and n memory cell transistors MC1 to MCn are completed.

By the above-described steps, a structure is obtained in which a part ofthe interface of the n-type impurity diffusion layer 16 and insulatingfilm 17 between the multi-layer gates of the select transistors disposedadjacent to each other is positioned to be lower than the interface ofthe p-type silicon substrate 10 and gate insulating film 11 of thechannel portion by the height of the stepped portion.

An enlarged view of the partial region of FIG. 3C formed as describedabove is shown in FIG. 3D. As shown in the drawing, a height d1 of thestepped portion, namely a depth between the silicon substrate 10 surfacecontacting the gate insulating film 11 and the impurity diffusion layer16 surface between the multi-layer gates, is larger than a filmthickness d2 of the gate insulating film 11 (d1>d2). That is, thesilicon substrate 10 is preferably etched so that the surface of atleast a part of the impurity diffusion layer 16 between the multi-layergates is deeper than the silicon substrate 10 surface contacting thegate insulating film 11 by d2 or more. This forms the side surface ofthe multi-layer gate of the select transistor to be vertical to thesurface of the silicon substrate 10. Additionally, when the height ofthe stepped portion is excessively large, an adverse influence resultingfrom a short channel effect becomes tremendous. Therefore, the heightneeds to be set to such an extent that the adverse influence is notexerted. Moreover, when the stepped portion is formed right under thecharge accumulation layer (a gate electrode of the select transistor)12, that is, the edge of the stepped portion is positioned right underthe charge accumulation layer 12, the film thickness of the gateinsulating film 11 substantially increases, and characteristics of theselect transistor are deteriorated. Therefore, the stepped portion ispreferably disposed outside the edge of the gate electrode 12. That is,a distance 11 to a stepped portion from the center of the gate electrode12 is preferably larger than a distance 12 to the edge from the centerof the gate electrode 12.

Next as shown in FIG. 3E, the contact barrier film 18 is formed of thesilicon nitride film on the silicon oxide film 17 and subsequently theinterlayer insulating film 19 is formed of the BPSG film on the contactbarrier film 18 to coat the multi-layer gate.

Subsequently, as shown in FIG. 3F, the photolithography technique andetching are used to form the contact hole C1 which reaches the drainregion of the select transistor ST1 and the contact hole C2 whichreaches the source region of the select transistor ST2. Moreover, in therespective contact holes C1, C2, the low-resistance polycrystal siliconfilm, or a contact material such as a metal film of tungsten is buried,and flatted so that the bit line contact plug 20 and source line contactplug 21 are formed.

Thereafter, the metal wiring layer 22 is formed on the interlayerinsulating film 19. A part of the metal wiring layer 22 forms the commonsource line SL. Furthermore, the interlayer insulating film 23 is formedon the interlayer insulating film 19, and the contact plug 25electrically connected to the bit line contact plug 20 is formed in theinterlayer insulating film 23. Moreover, the metal wiring layer 24 isformed on the interlayer insulating film 23 to constitute the bit lineBL so that the memory cell array region of the NAND-type flash memoryshown in FIGS. 2A and 2B is completed.

With the NAND-type flash memory according to the present embodiment, apart (middle portion) of the surface of the n-type impurity diffusionlayers (drain and source regions) 16, 16 in contact with the bit linecontact plug 20 and source line contact plug 21 is removed, in theselect transistors ST1, ST2. Consequently, the recess, namely the firststepped portion, is formed in the impurity diffusion layer 16.Therefore, a part of the interface of the bit line contact plug 20 andimpurity diffusion layer 16 or the interface of the source line contactplug 21 and impurity diffusion layer 16 is positioned to be lower thanthe interface of the channel region and gate insulating film 11 of theselect transistor by the height of the first stepped portion.

Subsequently, to obtain the constitution, the etching is performed sothat a part of the gate insulating film 11 and silicon substrate 10between the select transistors disposed adjacent to each other isetched. As a result, the side surface of the multi-layer gate of theselect transistor becomes substantially vertical to the siliconsubstrate 10 surface.

Therefore, it is possible to form the insulating film 17 and contactbarrier film 18 even in the etched portion of the silicon substrate 10.That is, when the contact plugs 20, 21 are formed, the insulating film17 and contact barrier film 18 are left (formed) between the multi-layergates of the select transistors ST1, ST2 and the contact plugs 20, 21.As a result, the multi-layer gates of the select transistors ST1, ST2and the contact plugs 20, 21 can be inhibited from electricallyshort-circuiting. Therefore, reliability of the select transistor isenhanced, and this can additionally contribute to enhancement of thereliability and manufacturing yield of the NAND-type flash memory.

Moreover, according to the constitution of the first embodiment, thestepped portion is formed only in the impurity diffusion layers 16, 16connected to the contact plugs 20, 21 in the select transistors ST1,ST2. That is, no stepped portion is formed in the impurity diffusionlayer 16 connected to the memory cell unit in the select transistorsST1, ST2. Therefore, the deterioration of characteristics of the selecttransistor, which is caused by the short channel effect by the surfaceof the n-type impurity diffusion layer 16 being deeper than the channelregion surface, can be minimized. Furthermore, no stepped portion isformed in the n-type impurity diffusion layers 16 of the memory celltransistor MC. Therefore, with respect to the memory-cell transistor MC,the characteristics can be prevented from being deteriorated by theabove-described cause. Moreover, the adverse influence by the shortchannel effect can be suppressed. As a result, further miniaturizationof the select transistor and memory cell transistor is possible.

Furthermore, for the above-described reason, the height of the steppedportion is larger than the thickness of the gate insulating film 11, andthe stepped portion is preferably disposed outside the edge of themulti-layer gate. Moreover, the gate insulating film 11 of the memorycell transistor and the gate insulating film 11 of the select transistorare substantially the same insulating film formed at the same time.Therefore, the same forming steps are used for these insulating films sothat-manufacturing cost can be reduced.

As described above, in the constitution of the present embodiment, atleast one of the bit line contact plug and common source line contactplug is formed in the multi-layer gate of the select transistor in theself-aligning manner. As a result, the multi-layer gate of the selecttransistor and the contact plug can be prevented from short-circuiting.Moreover, since the short channel effect of the select transistor andmemory cell transistor is not deteriorated, each transistor can furtherbe miniaturized.

Second Embodiment

The semiconductor memory device according to a second embodiment of thepresent invention will next be described with reference to FIGS. 4A and4B. FIG. 4A is a sectional view of the NAND-type flash memory accordingto the present embodiment along a bit line direction. Moreover, FIG. 4Bis a sectional view of a peripheral circuit region. The peripheralcircuit region is a region in which circuits other than the memory cell,such as the column selector and decoder circuit, are formed, or a regionin which an MOS transistor as a high withstand voltage system with awrite voltage applied thereto is formed. Moreover, in the secondembodiment, the structure described in the first embodiment is appliedto the MOS transistor in the peripheral circuit region. Additionally,since the structure of the memory cell array region is similar to thestructure described with reference to FIGS. 2A and 2B of the firstembodiment, the description thereof is omitted.

The peripheral circuit region is electrically separated from the memorycell array region by a element isolating region 30 disposed in thesilicon substrate 10. Moreover, the MOS transistor is formed on thesilicon substrate. The MOS transistor includes a gate electrode formedon the silicon substrate 10 with a gate insulating film 31 interposedtherebetween, and an impurity diffusion layer 34 formed in the siliconsubstrate 10. The gate electrode has a multi-layer gate structureincluding a semiconductor layer 32 formed on the gate insulating film31, a inter-gate insulating film 40 formed on the semiconductor layer32, and a semiconductor layer 41 formed on the inter-gate insulatingfilm 40. Moreover, the gate cap film 15 is formed on the semiconductorlayer 41. Additionally, in a region (not shown), the semiconductorlayers 32, 41 are electrically connected, and operate as a usual MOStransistor. That is, the gate electrode of the MOS transistorconstituting the peripheral circuit has a structure similar to that ofthe select transistor in the memory cell array region.

The impurity diffusion layer 34 has a recess, the bottom of which liesbelow the level of the surface of the silicon substrate 10. That is, atleast a part of the impurity diffusion layer 34 surface of the MOStransistor in the peripheral circuit is etched from the siliconsubstrate 10 surface by the etching. The recess caused the reference inlevel between a surface of the silicon substrate 10 right under the gateinsulating film 31 and a surface of a part of the impurity diffusionlayer 34 shall be referred to as “stepped portion”, hereinafter. Inparticularly, the stepped portion in the peripheral circuit is definedas “second stepped portion”. The height of the second stepped portion,namely depth between the silicon substrate 10 surface contacting thegate insulating film 11 and the impurity diffusion layer 34 surfacebetween the multi-layer gates, is the same as that of the first steppedportion.

Moreover, insulating films 43, 44 are formed on the impurity diffusionlayer 34 and multi-layer gate, and the interlayer insulating films 19,23 formed on the insulating film 44. Furthermore, contact plugs 36, 39which reach the source/drain region 34 of the MOS transistor, and metalwiring layers 37, 38 electrically connected to the plugs are disposed toform the peripheral circuit region.

According to the semiconductor memory device of the second embodiment,the stepped portion is formed not only in the select transistor in thememory cell array region but also in the MOS transistor in theperipheral circuit region. That is, in the MOS transistor in theperipheral circuit region, interfaces of-the contact plugs 36, 36 andimpurity diffusion layers 34, 34 are formed to be lower than theinterface of the channel region surface and gate insulating film 31 by aheight of the second stepped portion. Moreover, the second steppedportion is set to be of the same degree as that of the first steppedportion.

The above-described structure can be formed simultaneously withformation of the memory cell array region by the steps described in thefirst embodiment. That is, an RIE process described with reference toFIG. 3B is simultaneously performed not only in the memory cell arrayregion but also in the peripheral circuit region, so that the steppedportion in the memory cell and select transistor can simultaneously beformed. Moreover, the contact plug formation step described withreference to FIG. 3F is simultaneously performed in not only the memorycell array region but also the peripheral circuit region, so that thecontact plugs 20, 21, 36, 36 can simultaneously be formed.

That is, the MOS transistor of the peripheral circuit region is formedin the same step as that of the transistor in the memory cell arrayregion. Therefore, the second stepped portion of the same degree as thatof the first stepped portion is formed in not only the memory cell arrayregion but also the peripheral circuit region. Thereby, withoutperforming an extra step of forming a mask in the peripheral circuitregion, the structure described in the first embodiment can be realizedin the steps similar to the conventional steps.

Furthermore, even with the forming of the contact plugs 36 for example,by low-resistance polysilicon similarly as the contact plugs 20, 21 ofthe select transistor, No special trouble is caused, when the MOStransistor in the peripheral circuit region is, for example, a highwithstand voltage system transistor with the write voltage appliedthereto. This is because it is sufficient for the contact plug 36 tomainly fulfill a role of transmitting the voltage.

That is, according to the manufacturing method of the NAND-type flashmemory described above, the formation step of the contact hole openingand contact plug on the drain and source regions of the peripheraltransistor can be performed simultaneously with the formation step ofthe contact hole opening and contact plug of the select transistor.Therefore, the manufacturing cost of the NAND-type flash memory can bereduced.

Third Embodiment

The semiconductor memory device according to a third embodiment of thepresent invention will next be described with reference to FIG. 5. FIG.5 is a sectional view of the NAND-type flash memory according to thethird embodiment, and corresponds to a sectional view taken along thedirection of the line 2B-2B in FIG. 2A.

As shown in the drawing, for the constitution according to the thirdembodiment, the impurity diffusion layer 16 of the memory celltransistors MC1 to MCn has the recess, namely-the first stepped portion,the bottom of which lies below the level of the surface of the silicon,in the structure described in the first embodiment. That is, at least apart of the interface of the source/drain region 16 and insulating film17 of each of the memory cell transistors MC1 to MCn is formed to belower than the interface of the channel region (silicon substrate 10right under the charge accumulation layer 12) surface and gateinsulating film 11 by the height of the first stepped portion.

A manufacturing method of the NAND-type flash memory constituted asdescribed above will next be described with reference to FIGS. 6A to 6E.FIGS. 6A to 6E excluding FIG. 6C are sectional views successivelyshowing the manufacturing steps of the flash memory according to thethird embodiment, and FIG. 6C is an enlarged view of the partial regionof FIG. 6B.

First, the structure shown in FIG. 3A is obtained by the steps describedin the first embodiment.

Subsequently, as shown in FIG. 6A, the gate insulating film 11positioned between the multi-layer gates is etched, the siliconsubstrate 10 between the multi-layer gates is further etched, and therecess (first stepped portion) is formed in the silicon substrate 10.These steps are different from the steps of FIG. 3B of the firstembodiment in that the recess is formed not only between-the multi-layergates of the adjacent select transistors but also between themulti-layer gates of the adjacent memory cell transistors and betweenthe multi-layer gates of the select transistor and memory celltransistor disposed adjacent to each other. By the present steps, a partof the silicon substrate 10 between the multi-layer gates of the selecttransistor and memory cell transistor is removed, and the side walls ofthe multi-layer gates of the select transistor and memory celltransistor become substantially vertical to the silicon substrate 10surface.

Subsequently, as shown in FIG. 6B, the insulating film 17 is formed onthe upper surface and side wall of the multi-layer gate and on thesilicon substrate 10 between the multi-layer gates by the thermaloxidation method. Furthermore, n-type impurities are-injected into thesilicon substrate 10 between the multi-layer gates, so that the n-typeimpurity diffusion layer 16 constituting the source/drain region isformed. By the present steps, the select transistors ST1, ST2 and nmemory cell transistors MC1 to MCn are completed.

The enlarged view of the partial region of FIG. 6B formed as describedabove is shown in FIG. 6C. FIG. 6C shows the memory cell transistor. Theconstitution of the select transistor is similar to that of FIG. 3Ddescribed in the first embodiment.

As shown, the height d1 of the stepped portion, namely depth between thesilicon substrate 10 surface contacting the gate insulating film 11 andthe impurity diffusion layer 16 surface between the multi-layer gates,is preferably larger than the film thickness d2 of the gate insulatingfilm 11 (d1>d2). That is, the silicon substrate 10 is preferably etchedso that at least a part of the surface of the impurity diffusion layer16 between the multi-layer gates is deeper than the silicon substrate 10surface contacting the gate insulating film 11 by d2 or more. Moreover,when the stepped portion is formed right under the charge accumulationlayer 12, the film thickness of the gate insulating film 11 increases,and the characteristics of the memory cell are deteriorated. Therefore,the stepped portion is preferably disposed outside the chargeaccumulation layer 12 (13>14). That is, also with the memory celltransistor, conditions similar to those of the select transistordescribed in the first embodiment are preferably satisfied.

Subsequently, as described in the first embodiment with reference toFIG. 3E, the contact barrier film 18 and interlayer insulating film 19are formed to obtain the structure shown in FIG. 6D.

Subsequently, as described in the first embodiment with reference toFIG. 3D, the bit line contact plug 20 and source line contact plug 21are formed to obtain the structure shown in FIG. 6E.

Thereafter, as described in the first embodiment, the common source lineSL and bit line BL are formed, so that the memory cell array region ofthe NAND-type flash memory shown in FIG. 5 is completed.

According to the NAND-type flash memory of the third embodiment, theside surface of the multi-layer gate of the select transistor becomessubstantially vertical to the silicon substrate 10 surface. Therefore,similarly as the first embodiment, the multi-layer gates of the selecttransistors ST1, ST2 and the contact plugs 20, 21 can be inhibited fromelectrically short-circuiting. Therefore, the reliability of the selecttransistor is enhanced, and additionally the reliability andmanufacturing yield of the NAND-type flash memory can be enhanced.

Furthermore, according to the structure of the third embodiment, anelectric field concentration in a charge accumulation layer 12 edge issuppressed, and the reliability of the memory cell transistor can beenhanced. This respect will be described with reference to FIGS. 7A and7B. Both FIGS. 7A and 7B show an electric field distribution in thevicinity of the gate insulating film 11 in the silicon substrate 10positively biased with respect to the charge accumulation layer 12. FIG.7A is a sectional view of a memory cell transistor having the structureof the third embodiment, and FIG. 7B is a sectional view of the memorycell transistor having the conventional structure. Moreover, a pluralityof lines generated from the charge accumulation layer edge in thedrawing are contours indicating electric field intensities, and theelectric field weakens toward the outside.

As shown in FIG. 7A, according to the structure of the third embodiment,since the silicon substrate 10 (impurity diffusion layer 16) between themulti-layer gates is deep, the electric field does not easily expand tothe outside of the multi-layer gate. In the conventional structure shownin FIG. 7B, the electric field expands broadly to the outside of themulti-layer gate, as compared with the structure of the thirdembodiment. The broad expansion of the electric field to the outside ofthe multi-layer gate means that the electric field concentratesparticularly on the charge accumulation layer edge portion in thechannel length direction of the charge accumulation layer. However, inthe structure of the third embodiment, the expansion of the electricfield is inhibited. In other words, in the channel length direction ofthe charge accumulation layer, an electric field distribution is broughtclose to uniformity. Therefore, the electric field concentration in thecharge accumulation layer edge is prevented. As a result, electrons canbe prevented from being trapped into the gate insulating film 11 by thetunnel current, and the reliability of the memory cell can be enhanced.

Moreover, the stepped portion is disposed outside the chargeaccumulation layer 12. Therefore, the film thickness of the gateinsulating film 11 of the transistor is uniform along a channeldirection. Therefore, the above-described effect is obtained withoutdeteriorating the characteristics of the memory cell. This can easily berealized, when the insulating film 17 is formed on the multi-layer gateside walls and on the impurity diffusion layer 16 between themulti-layer gates by thermal-oxidation. This is because the oxidationquickly advances in the side wall portion of the multi-layer gate ratherthan on the impurity diffusion layer 16. This respect is similar to thefirst embodiment.

Furthermore, in the first embodiment, not to form the stepped portion inthe memory cell transistor, the resist needs to be formed on the memorycell transistor in the step of FIG. 3B. However, according to the thirdembodiment, since the stepped portion is also formed in the memory celltransistor, the resist is unnecessary. That is, one photolithographyprocess is unnecessary as compared with the manufacturing method of thefirst embodiment, and the manufacturing steps can be simplified.

FIGS. 8A to 8C are sectional perspective views successively showing somesteps of the manufacturing method of the flash memory according to amodification example of the third embodiment. Particularly, the etchingstep of the silicon substrate between the multi-layer gates describedwith reference to FIG. 6A is shown. In this step, a manufacturing methodof a self-aligning contact structure proposed in Jpn. Pat. Appln. KOKAIPublication No. 2002-057230 (U.S. patent application Ser. No.09/925,418), the entire contents (all pages) of these references beingincorporated herein by reference, is applied to the etching step of thesilicon substrate.

First, FIG. 8A is a sectional perspective view corresponding to FIG. 3A.As shown, each element isolating region STI includes a trench 27 formedin the silicon substrate 10, and an insulating film 26 such as a siliconoxide film with which the trench 27 is filled. Additionally, the uppersurface of the element isolating region STI is usually higher than thatof the silicon substrate 10. Therefore, when the bit line contact andcommon source line contact are to be formed with the self-aligningcontact, the contact barrier film 18 remains on a element isolatingregion STI side surface. To solve the problem, in the above-describedpatent applications, a technique of etching the insulating film 26 inthe state shown in FIG. 8A and lowering the upper surface of the elementisolating region STI is disclosed.

As shown in FIG. 8B, this technique is used to first etch the uppersurface of the insulating film 26 constituting the element isolatingregion STI by the etching of the silicon oxide film base, and to etchand remove the gate insulating film 11 between the multi-layer gates.

Here, even after the gate insulating film 11 is etched, the etching isfurther continued. Thereby, not only the insulating film 26 but also thesilicon substrate 10 between the multi-layer gates can be etched. As aresult, as shown in FIG. 8C, the silicon substrate surface between themulti-layer gates can be set to be lower than the silicon substratesurface right under the multi-layer gate. Additionally, the uppersurface of the insulating film 26 between the multi-layer gates can beset to be lower than that of the insulating film 26 right under themulti-layer gate. Of course, since the insulating film 26 under themulti-layer gate is not etched, the charge accumulation layers 12disposed adjacent to each other in a direction of control gate line CGare electrically separated.

With use of the above-described manufacturing method, since themulti-layer gate functions as the mask, for example, thephotolithography step, and a new step for etching the silicon substrate10 are not required. When only an etching time of the insulating film 26is simply lengthened, the silicon substrate 10 can be etched. Therefore,the flash memory according to the third embodiment can be manufacturedwithout complicating or elaborating the manufacturing steps. Moreover,in addition to the effect of the third embodiment, the effect describedin the above-described patent application can be obtained.

The steps describes with reference to FIGS. 8A to 8C can be performed inthe method according to the first embodiment, too. First, the structureof FIG. 8A is made. Then a resist of the same type as the resist 40shown in FIG. 3B is applied to the memory cells. Etching is performed bya silicon oxide-based etching treatment. Those parts of the siliconsubstrate 10 and the upper surface region of the insulating film 27which lie between the gate electrodes of the select transistors aretherefore removed. As a result, a structure according to the firstembodiment is provided. This structure has not only the advantagesspecified in the description of the first embodiment, but also theadvantages disclosed in the patent application identified above.

Fourth Embodiment

The semiconductor memory device according to a fourth embodiment of thepresent invention will next be described with reference to FIG. 9. FIG.9 is a sectional view of the NAND-type flash memory along the bit linedirection, and shows a sectional structure of the memory cell arrayregion and peripheral circuit region. The fourth embodiment correspondsto a combination of the second and third embodiments, and the structuredescribed above in the third embodiment is applied to the MOS transistorin the peripheral circuit region. Additionally, since the structure ofthe memory cell array region is similar to that of FIG. 5 describedabove in the third embodiment, the description is omitted. Moreover, thestructure of the peripheral circuit region is substantially the same asthat of the second embodiment, and will briefly be describedhereinafter.

As shown, in the peripheral circuit region, the MOS transistor is formedwhich includes the gate electrode 32 formed on the silicon substrate 10with the gate insulating film 31 interposed therebetween, an insulatingfilm 33, and the impurity diffusion layer 34 formed in silicon substrate10.

Similarly as the second embodiment, the impurity diffusion layer 34 ofthe MOS transistor has the recess, namely second stepped portion, thebottom of which lies below the level of the surface of the siliconsubstrate 10. The second stepped portion has the same height as that ofthe stepped portion.

According to the flash memory of the fourth embodiment, the transistorin the memory cell array region described above in the third embodiment,and the MOS transistor in the peripheral circuit can be formed by thesame steps. That is, the structure described in the third embodiment canbe realized in the steps similar to the conventional steps.

Fifth Embodiment

The semiconductor memory device according to a fifth embodiment of thepresent invention will next be described with reference to FIGS. 10A and10B. In the fifth embodiment, the first or third embodiment is appliedto a NOR-type flash memory. FIG. 10A is a plan view of the NOR-typeflash memory according to the fifth embodiment, and FIG. 10B is asectional view taken along a line 10B-10B.

As shown, similarly as the first and third embodiments, a plurality ofelement isolating regions STI are formed in strip forms in the p-typesilicon substrate (or a p-type well region) 10. Moreover, a regionbetween the adjacent element isolating regions STI constitutes theelement region AA.

In the element region AA, a plurality of memory cell transistors MC isformed. Each memory cell transistor MC includes the multi-layer gateformed on the silicon substrate with the gate insulating film (tunnelinsulating film) 11 interposed therebetween. The multi-layer gateincludes: the charge accumulation layer (FG) 12 formed on the gateinsulating film 11 and electrically separated for each memory cell; thecontrol gate 13 formed on the charge accumulation layer 12; theinter-gate insulating film 14 formed between the charge accumulationlayer 12 and control gate 13; and the gate cap film 15 formed on thecontrol gate 13. The impurity diffusion layers 16 of the n-type oppositeto the conductive type of the silicon substrate 10 in which the channelis formed are formed on the both sides of the multi-layer gate in thesilicon substrate 10. The impurity diffusion layers 16 function as thesource and drain regions (S, D). Each memory cell transistor and memorycell transistor are formed including the multi-layer gate and impuritydiffusion layer 16.

The memory cell transistors MC disposed adjacent to each other arearranged so as to share the source or drain region 16. The insulatingfilm 17 is formed on the multi-layer gate and impurity diffusion layer16, and the contact barrier film 18 is further formed on the insulatingfilm 17. Additionally, the impurity diffusion layer 16 between theadjacent multi-layer gates has recess, namely first stepped portion, thebottom of which lies below the level of the surface of the channelregion. That is, the interface of the silicon substrate 10 in which thechannel is formed and gate insulating film 11 is positioned higher thanthe interface of a part of the impurity diffusion layer 16 and theinsulating film 17.

Moreover, the interlayer insulating film 19 is formed on the contactbarrier film 18. Furthermore, in the interlayer insulating film 19, thecontact holes C1, C2 are formed which reach the drain region 16, sourceregion 16 of the memory cell transistor MC, and the bit line contactplug 20 and common source line contact plug 21 are formed in the contactholes C1, C2.

Furthermore, the metal wiring layer 22 is formed on the interlayerinsulating film 19. A part of the metal wiring layer 22 is connected tothe common source line contact plug 21, and functions as the commonsource line (SL). The interlayer insulating film 23 is further formed onthe interlayer insulating film 19, and the metal wiring layer 24 isformed on the interlayer insulating film 23. The metal wiring layer 24is connected to the bit line contact plug 20 via the contact plug 25,and functions as the bit line (BL).

Furthermore, the control gate 13 is connected in common in the deviceregions AA disposed adjacent to each other in a direction intersectingwith the element isolating region STI, and connected to the control gatelines CG1 to CGn in a region (not shown).

Moreover, the bit line BL is connected to the column selector (notshown), the control gate lines CG1 to CGn are connected to the rowdecoder (not shown), and the common source line SL is connected to theerase control circuit (not shown).

A manufacturing method of the NOR-type flash memory constituted asdescribed above is similar to that of the first or third embodiment, andthe description thereof is therefore omitted. It is of course possibleto use the method described in the modification example of the thirdembodiment.

As described above, according to the NOR-type flash memory of the fifthembodiment, as described in the first embodiment, the occurrence ofshort circuit between the charge accumulation layer 12 and bit linecontact plug 20 can be inhibited.

Moreover, the interface between the silicon substrate 10 and gateinsulating film 11 is higher than the interface between the plug 21 andsource region 16 and the between the plug 20 and drain region 16.Consequently, the electric field concentration in the chargeaccumulation layer 12 edge is prevented, and the reliability of thememory cell can be enhanced. In the NOR-type flash memory, electrons aredischarged to the source region 16 or silicon substrate 10 from thecharge accumulation layer 12 so as to rewrite data. That is, a biaswhich is positive with respect to the charge accumulation layer 12 isapplied to the silicon substrate 10, and the electrons are dischargedinto the silicon substrate 10. In this case, the electric fielddistribution in the vicinity of the gate insulating film 11 according tothe fifth embodiment is obtained as shown in FIG. 7A described above inthe third embodiment. That is, since the silicon substrate 10 (impuritydiffusion layer 16) between the multi-layer gates is deep, the electricfield does not easily expand to the outside of the multi-layer gate.Therefore, the electric field concentration in the charge accumulationlayer edge is prevented, and the electrons can be prevented from beingtrapped into the gate insulating film 11 by the tunnel current.Moreover, the bias positive with respect to the charge accumulationlayer 12 is applied to the source region 16, and thereby the electricfield distribution in the vicinity of the gate insulating film 11 indischarging the electrons to the source region 16 is a distribution inwhich only one edge of the charge accumulation layer 12 is noted in FIG.7A. Similarly in the conventional structure, only one edge of the chargeaccumulation layer of FIG. 7B may be noted. Even in this case, theregion in which the electric field is intense can be prevented fromexpanding to the outside of the multi-layer gate, and the electrons canbe prevented from being trapped into the gate insulating film 11 by thetunnel current.

As described above, even with the NOR-type flash memory, the effectsimilar to that of the third embodiment can be obtained.

Sixth Embodiment

The semiconductor memory device according to a sixth embodiment of thepresent invention will next be described with reference to FIG. 11. FIG.11 is a sectional view of the NAND-type flash memory along the bit linedirection.

For the NAND-type flash memory according to the sixth embodiment, in thestructure of FIG. 5 described above in the third embodiment, instead offorming the stepped portion, an insulating film 28 having a filmthickness larger than that of the insulating film 17 is formed on then-type impurity diffusion layer 16 between the multi-layer gates. Sincethe insulating film 28 is formed in and under the impurity diffusionlayer 16, the interface of the silicon substrate 10 and gate insulatingfilm 11 is positioned higher than the interface of a part of theimpurity diffusion layer 16 and insulating film 28.

A manufacturing method of the flash memory constituted as describedabove will next be described.

First the structure of FIG. 3A described in the first embodiment isformed. Subsequently, instead of etching the silicon substrate 10, thethermal oxidation is performed to form the insulating film (siliconoxide film) 17. In this case, the film thickness of the insulating film17 is controlled to be larger on the silicon substrate 10 than on thetop and side surfaces of the multi-layer gate. This can be realized,when impurities for increasing an oxidation rate, such as fluorine, areinjected into silicon substrate 10 between the multi-layer gates in thestructure of FIG. 3A. Then, the oxidation rapidly advances in the regionbetween the multi-layer gates as compared with the other regions.Therefore, the insulating film 17 in this region has a large filmthickness, and the insulating film 17 is formed in a deep portion of thesilicon substrate 10 (referred to as the insulating film 28).

Thereafter, the n-type impurities are injected into the siliconsubstrate 10 between the multi-layer gates, the n-type impuritydiffusion layer 16 forming the source, drain region is thereby formed,and the structure of FIG. 11 is obtained by the steps similar to thoseof the third embodiment.

Even in the structure according to the sixth embodiment, the steppedportion is formed so that the surface of the impurity diffusion layer 16between the multi-layer gates becomes lower than the silicon substrate10 right under the multi-layer gate, and therefore the effects similarto those of the third embodiment are obtained.

Of course, the insulating film 28 is preferably formed so that thethickness thereof in the impurity diffusion layer 16 is larger than thefilm thickness of the gate insulating film 11. Moreover, the end of theinsulating film 28 is preferably disposed outside the chargeaccumulation layer 12.

Seventh Embodiment

The semiconductor memory device according to a seventh embodiment of thepresent invention will next be described with reference to FIG. 12. FIG.12 is a sectional view of the NAND-type flash memory along the bit linedirection. The seventh embodiment is a combination of the first andsixth embodiments.

As shown, in the constitution of FIG. 11 described above in the seventhembodiment, the insulating film 28 is formed only on the n-type impuritydiffusion layer 16 between the adjacent select transistors.

Even with the present constitution, the effect similar to that of thefirst embodiment can be obtained.

Eighth Embodiment

The semiconductor memory device according to an eighth embodiment of thepresent invention will next be described with reference to FIG. 13. FIG.13 is a sectional view of the NOR-type flash memory along the bit linedirection. The eighth embodiment is a combination of the fifth and sixthembodiments.

In the constitution of FIG. 10B, instead of forming the stepped portionin the impurity diffusion layer 16, the insulating film 28 is formed.

Even with the present constitution, the effect similar to that of thefifth embodiment can be obtained. As described above, according to thesemiconductor memory device according to the first to fifth embodimentsof the present invention, in the MOS transistor in which at least one ofthe source and drain regions is connected to the contact plug, thestepped portion is formed in the impurity diffusion layer. The steppedportion causes the difference in level between the surface of theimpurity diffusion layer and the surface of the silicon substrate inwhich the channel is formed. Consequently, the part of the surface ofthe impurity diffusion layer is positioned lower than the channel regionsurface. Moreover, the contact plug is formed on the impurity diffusionlayer positioned lower than the channel region surface. Then, when apart of the impurity region surface is removed, the gate electrode sidesurface becomes substantially vertical to the semiconductor substratesurface. As a result, the contact plug can be prevented fromshort-circuiting with the gate electrode, and the reliability of thesemiconductor memory device can be enhanced.

Furthermore, according to the semiconductor memory device of the firstand second embodiments, in the NAND-type flash memory, the steppedportion is formed only on the select transistor, and is not formed inthe memory cell transistor. As a result, the characteristicdeterioration caused by the short channel effect in the memory celltransistor can be prevented.

Furthermore, according to the semiconductor memory device of the thirdand fourth embodiments, even in the MOS transistor in which both thesource and drain regions are not connected to the contact plug, such asthe memory cell transistor of a NAND-type memory, the stepped portion isformed. As a result, the electric field distribution in the channellength direction of the charge accumulation layer in the memory celltransistor nearly becomes uniform, and the electric field concentrationin the charge accumulation layer edge is prevented. Therefore, theelectrons can be prevented from being trapped into the gate insulatingfilm by the tunnel current, and the reliability of the memory celltransistor can be enhanced. Of course, as described in the first andsecond embodiments, the stepped portion formed in the memory celltransistor needs to be formed to such an extent that the adverseinfluence caused by the short channel effect is not exerted.

Furthermore, according to the semiconductor memory device of the secondand fourth embodiments, the stepped portion is formed not only in thememory cell array region but also in the MOS transistor in theperipheral circuit region. Therefore, the memory cell array region andthe MOS transistor of the peripheral circuit region can simultaneouslybe formed in the same steps. Therefore, the manufacturing steps can besimplified, and manufacturing cost can be reduced.

Furthermore, according to the semiconductor memory device of the sixthto eighth embodiments, instead of forming the stepped portion in thefirst to fifth embodiments, the insulating film is formed so as to sinkin the impurity diffusion layer. Even with these embodiments,-theeffects similar to those of the first to fifth embodiments are obtained.Additionally, the constitution in which the insulating film 28 is formedas described in the sixth to eighth embodiments may be applied not onlyto the memory cell array region but also to the peripheral circuitregion described in the second and fourth embodiments.

In addition, the embodiments have been described above in which theheight of the stepped portion is not less than that of the gateinsulating film. However, even when the height is smaller than that ofthe gate insulating film, the similar effects are of course obtained. Ina viewpoint of the prevention of the electric field concentration, theheight of the stepped portion is preferably large. However, when thestepped portion is excessively large, the short channel effect issupposed to remarkably appear. Therefore, it is necessary to set thedepth to such an extent that such adverse influence is not generated.Additionally, the embodiments of the present invention are not limitedto the NAND-type and NOR-type flash memories described above in theembodiments, and can also be applied to an AND-type flash memory andgeneral EEPROM. Furthermore, the embodiment of the present invention canbe applied to general semiconductor memory devices in which theelectrons are transmitted/received via the insulating film by an appliedhigh voltage in order to rewrite the data, further to generalsemiconductor memories including nonvolatile memory cell transistors andselect transistors, and to memory embedded devices.

The embodiments of the present invention as shown in FIGS. 2A-13 havemany different implementations. A few of these implementations are shownin FIGS. 14-20.

In one example, shown in FIG. 14, a memory card 60 includes thesemiconductor memory device 50 as disclosed in one of the FIGS. 2A-13.As shown in FIG. 14, the memory card 60 is operable to receive/outputpredetermined signals and data from/to an external device (not shown).

A signal line (DAT), a command line enable signal line (CLE), an addressline enable signal line (ALE) and a ready/busy signal line (R/B) areconnected to the memory card 60 having the semiconductor memory device50. The signal line (DAT) transfers data, address or command signals.The command line enable signal line (CLE) transfers a signal, whichindicates that a command signal is transferred on the signal line (DAT).The address line enable signal line (ALE) transfers a signal, whichindicates that an address signal is transferred on the signal line(DAT). The ready/busy signal line (R/B) transfers a signal, whichindicates whether the memory device 50 is ready, or not.

Another exemplary implementation is shown in FIG. 15. The memory cardshown in FIG. 15 differs from the memory card presented in FIG. 14 inthat the memory card 60 of FIG. 15 includes, in addition to the memorydevice 50, a controller 70 which controls the semiconductor memorydevice 50 and receives/transfers predetermined signals from/to anexternal device (not shown).

The controller 70 includes an interface unit (I/F) 71, 72, amicroprocessor unit (MPU) 73, a buffer RAM 74 and an error correctioncode unit (ECC) 75. The interface unit (I/F) 71, 72 receives/outputspredetermined signals from/to an external device (not shown) and thesemiconductor memory device 50, respectively. The microprocessor unit 73converts a logical address into a physical address. The buffer RAM 74stores data temporarily. The error correction code unit 75 generates anerror correction code. A command signal line (CMD), a clock signal line(CLK) and a signal line (DAT) are connected to the memory card 60. Itshould be noted that the number of the control signal lines, bit widthof the signal line (DAT) and a circuit construction of the controller 70could be modified suitably.

Another exemplary implementation is shown in FIG. 16. As can be seenfrom FIG. 16, a memory cardholder 80 is provided for receiving a memorycard 60 having a memory device 50 as discussed in connection with FIGS.2A-13. The cardholder 80 is connected to an electronic device (notshown) and is operable as an interface between the card 60 and theelectronic device. The cardholder 80 may perform one or more of thefunctions of the controller 70 described in connection with FIG. 15.

Another exemplary implementation will be explained with reference toFIG. 17. FIG. 17 shows a connecting apparatus operable to receive amemory card or a cardholder, either of which includes the memory device.The memory card or cardholder is insertable in the connecting apparatus90 and is electrically connectable to the apparatus. The connectingapparatus 90 is connected to a board 91 via a connecting wire 92 and aninterface circuit 93. The board 91 contains a CPU (Central ProcessingUnit) 94 and a bus 95.

Another exemplary implementation is shown in FIG. 18. As shown in FIG.18, a memory card 60 or a cardholder 80, either of which includes thememory device, is inserted and electrically connectable to a connectingapparatus 90. The connecting apparatus 90 is connected to a PC (PersonalComputer) 300 via connecting wire 92.

Another exemplary implementation is shown in FIGS. 19 and 20. As shownin FIGS. 19 and 20, a semiconductor memory device 50 as described inconnection with FIGS. 2A-13 and other circuits such as ROM (read onlymemory) 410, RAM (random access memory) 420 and CPU (central processingunit) 430 are included in an IC (interface circuit) card 500. The ICcard 500 is connectable to an external device via a plane terminal 600that is coupled to an MPU (micro-processing unit) portion 400 of thecard 450. The CPU 430 contains a calculation section 431 and a controlsection 432, the control section 432 being coupled to the memory device50, the ROM. 410 and the RAM 420. Preferably, the MPU 400 is molded onone surface of the card 500 and the plane connecting terminal 600 isformed on the other surface.

Other implementations are readily discernable to one of ordinary skillin the art when the present description is read in view of thedescription in U.S. Pat. No.: 6,002,605, which is incorporated herein byreference.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventionconcept as defined by the appended claims and their equivalents.

1-71. (canceled)
 72. A semiconductor memory device comprising: a firstselect transistor formed on an upper surface of a substrate and having afirst multi-layer gate; a second select transistor formed on the uppersurface of the substrate and having a second multi-layer gate; a firststepped portion formed by etching the substrate adjacent to the firstmulti-layer gate of the first select transistor such that the firststepped portion forms a cavity in the upper surface of said substrate; asecond stepped portion formed by etching the substrate adjacent to thesecond multi-layer gate of the second select transistor such that thesecond stepped portion forms a cavity in the upper surface of saidsubstrate; a first contact plug formed in a region of the first steppedportion, the first contact plug being partially in contact with thefirst stepped portion; a second contact plug formed in a region of thesecond stepped portion, the second contact plug being partially incontact with the second stepped portion; a first insulating film formedbetween the first contact plug and the first multi-layer gate, the firstinsulating film being partially in contact with the first steppedportion; a second insulating film formed between the second contact plugand the first multi-layer gate, the second insulating film beingpartially in contact with the second stepped portion; a memory cell unitincluding at least one memory cell transistor formed between the firstselect transistor and the second select transistor, each memory cellincluding an impurity diffusion layer; and a third stepped portionformed by etching the substrate adjacent to each memory cell such thatat least part of the third stepped portion forms a cavity in the uppersurface of said substrate.
 73. A semiconductor memory device comprising:a memory cell unit including at least one memory cell transistor, thememory cell transistor including: first and second semiconductor regionswith a first conductivity which are formed apart from each other in asurface of a semiconductor layer with a second conductivity opposite tothe first conductivity; a multi-layer gate electrode which is formed onthe semiconductor layer between the first and second semiconductorregions with a first gate insulating film interposed therebetween, themulti-layer gate electrode including a charge accumulation layer and acontrol gate; and a first insulating film formed on the first and secondsemiconductor regions; a select transistor which selects the memory cellunit, the select transistor including: third and fourth semiconductorregions with the first conductivity which are formed apart from eachother in a surface of the semiconductor layer; a first gate electrodewhich is formed on the semiconductor layer between the third and fourthsemiconductor regions with a second gate insulating film interposedtherebetween; a second insulating film formed on the third semiconductorregion, at least a part of an interface between the third semiconductorregion and second insulating film being positioned low to have a firststepped portion with respect to an interface between the semiconductorlayer and second gate insulating film; a third insulating film formed onthe fourth semiconductor region, at least a part of an interface betweenthe fourth semiconductor region and the third insulating film beingpositioned low to have a second stepped portion with respect to aninterface between the semiconductor layer and second gate insulatingfilm; a first contact plug which is formed in a region of the secondstepped portion, the first contact plug being partially in contact withthe second stepped portion and being electrically connected to one ofbit and source lines, a part of the third insulating film beingpartially in contact with the second stepped portion; and a memory cellarray in which a plurality of memory cell units and select transistorsare formed.
 74. The semiconductor memory device according to claim 73,wherein at least a part of the interfaces between the firstsemiconductor layer and first insulating film and between the secondsemiconductor layer and first insulating film is positioned low to havethe first stepped portion with respect to the interface between thethird semiconductor layer and first gate insulating film.
 75. Thesemiconductor memory device according to claim 73, wherein a height ofthe first stepped portion and a height of the second stepped portion arelarger than a thickness of the second gate insulating film.
 76. Thesemiconductor memory device according to claim 74, wherein a height ofthe first stepped portion in the memory cell transistor is larger than athickness of the first gate insulating film.
 77. The semiconductormemory device according to claim 73, wherein the first stepped portionand the second stepped portion are positioned outside an edge of thefirst gate electrode.
 78. The semiconductor memory device according toclaim 74, wherein the first stepped portion in the memory celltransistor is positioned outside an edge of the multi-layer gateelectrode.
 79. The semiconductor memory device according to claim 73,wherein the charge accumulation layer transmits/receives electrons withat least one of the first and second semiconductor regions and thesemiconductor layer via the first gate insulating film to write data.80. The semiconductor memory device according to claim 73, wherein thememory cell unit is a NAND cell unit in which a plurality of memory celltransistors is connected in series.